Mark Twain (2014). “Mark Twain on Common Sense: Timeless Advice and Words of Wisdom from America's Most-Revered Humorist”, p.53, Skyhorse Publishing, Inc.
Attributed in Reader's Digest, Dec. 1948. Commonly attributed to Twain, but the Stevens Point (Wis.) Daily Journal, 19 Dec. 1913, printed the following without attribution to any named individual: "Golf, of course, has been defined as a good walk spoiled."